1. Field of the Invention
This invention relates to froth flotation separation installations and more particularly those froth flotation separation installations wherein a mixture of solid particles is separated into a float product and a non-float (or sink) product during transit as an aqueous slurry through sequential flotation zones in which the aqueous slurry is repeatedly agitated and in which gaseous bubbles are introduced adjacent to the bottom of each flotation zone. The float product passes upwardly through the aqueous slurry with the gaseous bubbles and is collected as a froth above the upper surface of the aqueous slurry. The aqueous slurry which is not recovered as a float product is recovered as a non-float (or sink) product.
2. Description of the Prior Art
Froth flotation installations are widely used in the mineral separation industries for separating solid raw materials into a useful product and a waste product according to the difference in the physical properties of the materials, especially the surface properties of the raw materials. Froth flotation is extensively used to concentrate coal or mineral sulfides and oxides. Finely ground ores or coal have particles with different surface properties with respect to water, i.e., some particles are hydrophilic and some particles are hydrophobic. In some ores, all of the particles are hydrophilic in varying degrees. The differential in hydrophilic characteristics permits separation of the more hydrophilic particles from the less hydrophilic particles. The finely divided fresh particles are agitated in water with air bubbles. The bubbles and the particles combine and rise to the surface of the aqueous slurry as a frothy concentrate which can be skimmed for collection above the level of the aqueous slurry of unrecovered particles. The more hydrophilic particles remain in the aqueous slurry and are recovered as a sink product.
Various chemical reagents are added to the froth flotation installations to improve the recovery. These reagents are
frothing agents which alter the surface tension of the water and thus promote small bubble formation;
collectors which improve the attachment of particles to bubbles and assist in forming a stable froth;
activators which improve the performance of the collectors;
depressants which selectively interfere with the effectiveness of the collectors.
While the present invention is applicable to separation of mineral ores, its application to coal separation will be discussed in detail for simplicity. Coal which is to be separated in froth flotation equipment is customarily ground to fine particle size, for example, 0.75 millimeters. The fine particles of coal are delivered as an aqueous slurry as raw fine coal or obtained from prior separation equipment (e.g., centrifugal separators such as hydrocyclones, screens, etc.). The function of the froth flotation process is to recover two distinct products. The float product contains most of the combustible ingredients of the raw coal and generally has a reduced sulfur content and a reduced ash content when compared with the raw coal. The non-float (or sink) product contains less combustible ingredients, more ash ingredients and generally more sulfur ingredients than the raw coal.
Typically the froth flotation process is carried out in a number of sequential flotation cells wherein an aqueous slurry of raw coal solids is introduced into a first froth flotation cell and subjected to agitation with rising gas bubbles to permit flotation of the more hydrophobic particles for recovery as a froth above the liquid surface of the aqueous slurry of unrecovered solids. The aqueous slurry of unrecovered solids moves from the first flotation zone to a second flotation zone where the slurry is again agitated with freshly created upwardly rising gas bubbles to effect further separation of the more hydrophobic particles. An aqueous slurry of unrecovered, non-float solids passes from the second flotation zone through succeeding intermediate flotation zones, if any, where the agitation of the aqueous slurry of unrecovered solids with upwardly rising freshly created gas bubbles is repeated and froth containing the more hydrophobic particles is recovered above the level of the slurry of unrecovered, non-float solids.
The aqueous slurry of unrecovered, non-float solids from the last of the intermediate flotation zones is delivered to the last flotation zone where a final agitation of the aqueous slurry with freshly created rising gas bubbles is carried out. The more hydrophobic remaining particles rise upwardly along with gas bubbles in the last flotation zone. An aqueous slurry of unrecovered, non-float solid particles from the last flotation zone is separately recovered as the non-float (or sink) product of the process. One of the shortcomings of the sequential froth flotation separation process is the amount of energy required to agitate the aqueous slurry of unrecovered solids and to generate fresh gas bubbles near the bottom of each individual flotation zone. A typical agitation/bubble formation involves a motor-driven mechanical agitator in the central region of each individual flotation zone for creating agitation and aeration in the aqueous slurry. The energy required in each of the flotation zones is appreciable and significantly affects the cost of the separation process.
Another phenomenon associated with froth flotation is the increasing difficulty of establishing efficient separation in succeeding froth flotation zones. In the initial froth flotation zone, the incoming raw coal solids have a substantial fraction of particles which will enter into the float product. However as these float product particles are removed from the first flotation zone, there are fewer float product particles remaining in the aqueous slurry of unrecovered solids which is delivered to each succeeding flotation zone. Each succeeding flotation zone requires greater energy to create additional agitation and aeration to achieve the more difficult separations. The recent history of froth flotation installations shows that the energy of the agitation apparatus is not increased in response to this need for progressively higher energy from feed to tailings. Instead, common practice has been to install long lines of smaller agitation apparatus, each with about equal energy requirements. See U.S. Pat. No. 3,400,818. By providing more froth flotation zones, each with a smaller agitator apparatus, high separation efficiencies can be maintained but at a sacrifice of greatly increased length of the froth flotation installation with rsulting higher cost. Hence, to reduce the higher costs, the current practice is to increase the size of the agitator-aerators, and to increase the size of the flotation zones, but to shorten the length of the froth flotation installation. The reduction in overall length decreases investment expenses and reduces building requirements and energy consumption. However, separation efficiency, resulting from shorter length and lower energy, may be adversely affected.
One of the devices heretofore employed in froth flotation units is a vortex chamber which receives pressurized liquid from a tangential entry pipe and delivers a single bottom liquid product with great turbulence. The vortex chamber also functions as an aspirator for flotation gas and hence comprises a single unit which achieves the requisite agitation and flotation gas bubble formation within a froth flotation cell. The aqueous slurry introduced into such vortex chambers heretofore is a side stream of aqueous slurry of unrecovered solids drawn from one of the intermediate froth flotation zones or from the last froth flotation zone.
A vortex chamber has a cylindrical body, an inverted conical frustum base, a top central pipe extending into the interior and at least one tangential feed conduit. Liquids at elevated pressure are delivered through the tangential feed conduit into the cylindrical body to create a vortex therein. All of the liquids are discharged through an opening at the bottom of the conical frustum base. Gases are aspirated into the vortex through the top control pipe so that the discharged liquids contain dispersed gas bubbles. Vortex chambers also are called aeration chambers or aeration and agitation chambers. Multiple tangential feed conduits might be employed for the vortex chamber as suggested in U.S. Pat. No. 4,090,956.
Heretofore the aqueous slurry of raw coal solids has been delivered to the first froth flotation zone from a collector box which is reasonably non-turbulent. The flow velocity of the incoming aqueous slurry of raw solids has been intentionally dissipated in the relatively non-turbulent collector box.
The expression "raw coal" in this specification includes freshly mixed coal, also coal which has received some preliminary separation processing and also coal which is recovered from silt ponds and similar accumulations of fine coal previously considered to be unsuitable for recovery.